Continuous Coating of Pellets

ABSTRACT

A continuous dosage form coating apparatus uses vibrational impulses to maintain a dosage forms in a fluid state to expose them to a coating material atomized by spraying.

FIELD OF THE INVENTION

This invention relates to systems, methods and apparatuses for applyingcoatings to pellets, and has particular utility in coatingpharmaceutical dosage forms and other pellet-shaped materials.

BACKGROUND OF THE INVENTION

Dosage forms, such as compressed tablets, chewable tablets, fastdissolving tablets, capsules, softgels, and gelcaps are known in thepharmaceutical arts.

Production of these dosage forms is often carried out in steps. Someproduction steps are continuous, and others are carried out as “batch”processes. The distinction is that, in a continuous production step, thedosage forms can be fed to and withdrawn from a processing stagecontinuously, usually without any time limit, whereas in batchprocessing, a quantity of dosage forms is fed to a processing stage, andthen processed and withdrawn.

In the manufacture of many of these dosage forms, it is common to coatpellets with films or with layers of films. In the case ofpharmaceutical products, the coatings can have a number of purposes. Thecoatings can be cosmetic, pharmaceutically active, or otherwisefunctional.

For example, a coating can be used to prevent a portion of a drug frombeing released in the form of dust. It can be used to mask an unpleasantodor or taste of the active drug, or of a filler or binder. It can beused to facilitate swallowing by providing the dosage form with asmoother and less absorbent outer layer. A coating resistant to gastricfluids can be used to prevent premature digestion of the contents of adosage form. A coating can also control the rate of absorption of thedrug by the small intestine. A coating can also be used to provide adose of another drug in combination with the base dosage form. Finally,a coating can improve the appearance of the tablet, impart a distinctivecolor to the tablet for identification, and provide a printable surface.

Dosage forms are often coated using machines which spray a liquefiedcoating material onto the surfaces of the dosage forms while the dosageforms are in motion within a container. Examples of typical liquefiedcoating materials, include hydroxypropylmethylcellulose (HPMC) andstarch-based materials. These coating materials may or may not includepigment,

Two common types of machines tumble tablets within a drum that rotatesabout a horizontal axis during the spraying process.

The coating step for pharmaceutical volume products is most often abatch process, and in the most commonly used batch process, a perforatedpan coating machine is used. The perforated pan machine includes arotating, perforated drum which rotates about a horizontal axis within ahousing, and further includes a plurality of nozzles positioned withinthe drum. The nozzles create a spray of coating material within the drumso that dosage forms located within the drum will tumble about into andout of the spray pattern and, over a period of time, accumulate acoating on their surfaces.

Appropriate ducting is used to direct air through the housing of theperforated pan machine so that it passes through the perorated drum andreaches the dosage forms tumbling therein. The perforations of the drumexpose the tumbling dosage forms to a current of air, resulting in moreuniform drying. The drum further includes baffles, which enhance mixingof the dosage forms in order to improve the distribution of the materialbeing sprayed onto the tablets.

Unfortunately, batch coating has drawbacks. For example, each of thevarious apparatuses employed in batch coating is housed in a separateclean room that must meet standards set by the Food and DrugAdministration. This requires a relatively large amount of capital interms of space and machinery.

Batch coating processes are also difficult to control because thecontrol algorithms attempt to control the process toward adifficult-to-define endpoint, rather than control a continuous processwhere parameters can be controlled using feedback.

Heretofore, batch coating processes have inhibited manufacturers frominterconnecting process stages, and from flexibly interconnectingcontinuous stages of various kinds and capabilities to meetmanufacturing requirements. A process that would increase and streamlineproduction rates by coupling continuous processing stages in line wouldprovide many economic benefits, including a reduction in the size offacilities needed for mass production of pharmaceutical products.Generally, it would be desirable to create a continuous coating processfor the formation of tablets and other dosage forms, so that linkagescan be made with other similar or different operations such as tabletcompression. By making such linkages, it will be possible to carry outdosage form production in an overall continuous process.

Continuous coating processes for dosage forms also exist. An example isthe model CC-3015 continuous coater made by O′Hara Technologies ofRichmond Hill, Ontario, Canada. These continuous coating processesutilize rotating cylinders, and are generally limited to relativelylarge throughput volumes. The reason is that there are practical limitson how close the spraying systems can be to the bed of pellets to becoated. The required spray-to-bed distance, and also the need toaccommodate monitoring sensors in the vicinity of the product beingcoated, imposes a limit on how small the diameter of the cylinder canbe, and therefore limits the ability of a manufacturer to scale down acontinuous coating process to lower throughput volumes. Thus, whilecontinuous coating is useful in the production of non-prescriptionproducts such as calcium supplements, antacids, and other products soldin high volumes, it is difficult to scale down a continuous coatingmachine to make it practical for use with lower throughput volumes suchas in the case of most prescription drugs. Consequently, coating of manyprescription drugs is still carried out in a batch mode.

Another problem common to the existing batch coating machines andcontinuous coating machines is that shear forces and stressesencountered by tablets in these machines can cause splitting or chippingof tablets, especially multi-layer tablets and tablets having lessphysically robust formulations.

SUMMARY OF THE INVENTION

The invention provides for continuous exposure of pellets, e.g., dosageforms such as medicinal tablets, to the spray of a coating apparatuswithout the use of a rotating cylinder, and therefore without anyminimum cylinder size limitation. Process monitoring and feedback isalso made easier because of the elimination of the constraints imposedby the rotating cylinder.

Briefly, in the case of a rotating cylinder, the spraying apparatus mustbe inside the cylinder, and may therefore be too close to the bed ofpellets in the case of a cylinder having a relatively small-diameter.The invention avoids the limitation on the height of the sprayingapparatus by utilizing vibration to induce rolling motion of a bed ofpellets in a trough. The trough can be open at its top, or, if enclosed,can be of a shape such that the spray nozzles can be disposed at therequired distance from the pellet bed. The trough can have, but does notnecessarily have, an arc-shaped transverse cross-section.

Vibratory finishing has been used for a long time for materialtreatment, e.g., for particle milling. In general, vibratory finishingprocesses utilize mixtures of particles of different masses, e.g.,abrasives and parts to be finished. However, so far as I am aware, nosuccessful coating system has been utilized to coat pellets, all havingsubstantially the same mass, in a bed in which rolling circulation ofthe bed is induced by vibratory motion rather than by rotation.

More particularly, in the continuous pellet coating apparatus inaccordance with the invention, an elongated trough has an inlet and anoutlet separated from each other by a longitudinal distance along thedirection of elongation of the trough. A feeder is provided forcontinuously delivering pellets to be coated into the trough at alocation adjacent the inlet, and thereby establishes a bed of pellets inthe trough which travel longitudinally along the trough as pellets aredelivered by the feeder. A weir, disposed in the trough adjacent theoutlet, has an edge over which pellets are discharged as pellets to becoated are delivered to the trough by the feeder. The weir establishes amaximum level of the bed of pellets in the trough. At least one spraynozzle is disposed above the maximum level of bed of pellets in thetrough, and each spray nozzle is arranged to direct a spray of liquefiedcoating material toward pellets in the trough at an intermediatelocation between the inlet and outlet. A mechanical energy-impartingsource, connected to the trough, vibrates the trough, and therebymaintains the bed of pellets therein in a substantially fluidized state,while rotating the bed so that substantially all of the pellets becomeexposed to the spray of liquefied coating material as they travel alongthe trough.

In one preferred embodiment of the coating apparatus, the elongation ofthe trough is linear, the trough is pivoted on an axis substantiallyparallel to its direction of elongation, and the mechanicalenergy-imparting source is connected to the trough at a location spacedlaterally from the pivot axis, and arranged to apply impulse componentsto the trough as moments about the pivot axis. These impulse componentshave sufficient intensity to rotate the bed of pellets in the trough, sothat substantially all of the pellets reach the surface of the bed andare exposed to the spray of liquefied coating material at times duringtheir travel from the inlet to the outlet.

Because there is no need for a rotating cylinder, the coating apparatuscan be scaled down to a relatively small size while allowing all or partof the spray nozzle assembly to be positioned at any distance above thepellets in pellet bed. Another advantage of the elimination of therotating cylinder is that the bed of pellets does not need to travelalong a straight path. The bed can travel in a circular, arcuate, orhelical path, for example.

The mechanical energy-imparting source is preferably arranged to applyimpulse components having sufficient intensity to rotate the bed ofpellets in the trough, so that substantially all of the pellets reachthe surface of the bed and are exposed to the spray of liquefied coatingmaterial at times during their travel from the inlet to the outlet. Theapparatus also preferably includes means for adjusting impulseintensity, amplitude, direction, and/or frequency.

In an embodiment of the invention, at least one additional weir, andpreferably a series of longitudinally spaced weirs, is provided in thetrough between the inlet and outlet to achieve improved uniformity ofthe residence time of the pellets in the trough.

In accordance with another aspect of the invention, an array of airholes is formed in the trough, and an enclosure cooperates with thetrough to provide an air plenum. The enclosure can be situated eitherabove or below the trough, and a blower is connected to the air plenumfor causing air to flow through the air holes and through the bed ofpellets. Optionally, enclosures can be arranged to provide air plenumsboth above and below the trough. The air can flow through the bed whilethe spray of liquefied coating material is directed toward the pellets,and can also flow through the bed while coating material is not beingsprayed. Preferably, the air flows though the bed of pellets in thetrough, and, from within the bed of pellets, outward from the troughthrough the air holes. However, it is also possible for the air totravel in the opposite direction, that is, into the pellet bed throughthe air holes in the trough.

The air holes can be incorporated into one or more intermediate weirs,which can be made hollow. For example, the coating apparatus can includeat least one additional, hollow, weir in the trough between the inletand outlet. The additional weir can have walls facing the inlet of thetrough and the outlet of the trough, and one or both of the walls canhave an array of air holes. An enclosure cooperating with the troughforms an air plenum, and a blower connected to the air plenum can beused to cause air to flow through the air holes in the hollow weir, andthrough the bed of pellets. Preferably, the air flows through the bed ofpellets and outward through the air holes in the hollow weir. However,as in the previously described embodiment, in which air holes are formedin the bottom of the trough, the air can be made to flow in the oppositedirection. In the previously described embodiment, the circulation ofpellets is influenced both by the vibration of the trough and by theflow of air. In the case of a hollow weir, however, the air flowsthrough the air holes horizontally or nearly horizontally, and has lesseffect on the circulating movement of the pellets.

In a preferred embodiment of the coating apparatus, a monitor isprovided for monitoring a condition of the operation of the coatingapparatus, and a control, responsive to the monitor, adjusts one or moreoperating parameters of the apparatus. The operating parameters that canbe adjusted include the rate at which the feeder delivers pellets to becoated to the trough, air temperature, air flow, vibration amplitude,vibration intensity, vibration frequency, vibration direction, liquefiedcoating material spray rate and spray pressure.

Whether the trough is linear or curved, for example circular, arcuate,or helical, the mechanical energy-imparting source is arranged to applyimpulse components to the trough, which, in turn, apply a force to thebed of pellets exceeding the force of gravity. The impulse componentsare oriented and timed to cause the bed of pellets in the trough torotate continuously, so that substantially all of the pellets reach thesurface of the bed and are exposed to the spray of liquefied coatingmaterial at times during their travel from the inlet to the outlet.

Where air flow is utilized in combination with vibration, a monitor mayalso be included for monitoring a condition of the operation of thecoating apparatus, and a control, responsive to the monitor, may beprovided for adjusting one or more of the previously mentioned operatingparameters.

Where air flow is utilized, and especially where the air flow has asubstantial vertical component affecting the movement of pellets in thepellet bed, the impulse components should apply a force to the pelletbed exceeding the resultant of the forces applied to the bed by gravityand by air flowing through the bed. Here again, the impulse componentsshould be oriented and timed to cause the bed of pellets in the troughto rotate continuously, so that substantially all of the pellets reachthe surface of the bed and are exposed to the spray of liquefied coatingmaterial at times during their travel from the inlet to the outlet.

Another, but related, aspect of the invention is a method ofcontinuously coating pellets. The method comprises four operations thatare carried out continuously. Pellets are fed continuously into anelongated trough at a first location along the length of the trough, anda bed of pellets is maintained in the trough, the bed continuouslymoving longitudinally in the trough. A spray of liquefied pellet coatingmaterial is directed toward the bed of pellets in the trough at anintermediate location along the length of the trough. The trough isvibrated so that the bed is substantially fluidized and the bed ofpellets is caused to rotate so that substantially all of the pellets areexposed to the spray at times during their travel along the trough.Coated pellets are discharged over a weir in the trough at a dischargelocation longitudinally spaced from the first location. The coatedpellets are collected as they are discharged. The intermediate location,at which the spray of coating material is directed toward the bed ofpellets, is between the first location and the discharge location.

Where the elongation of the trough is linear, vibration of the troughmay be carried out by applying impulse components to the trough asmoments about an axis parallel to the direction of elongation of thetrough, the impulse components having sufficient intensity to rotate thebed of pellets in the trough.

The liquefied pellet coating material can be sprayed toward the bed ofpellets in the trough by one or more spray nozzles located at asufficient distance from the pellets that the coating material reachesthe pellets in a partially dried condition such that solvent in thecoating material does not damage the surfaces of the pellets.

To make the pellet residence time in the trough more uniform, the bed ofpellets can be moved along the trough over an additional weir at anintermediate location, or a series of longitudinally spaced weirs.

To minimize waste of coating material, or to aid in distribution of thecoating material, air is also preferably caused to flow through the bedof pellets. The air can be caused to flow through the bed of pelletseither inwardly or outwardly through an array of openings in the trough.The openings can be formed in one or more hollow weirs at intermediatelocations along the length of the trough, and in that case, the flow ofair through the openings can be horizontal or nearly horizontal. Hereagain the flow of air can be either into the trough, or outward from thetrough, through the array of openings in the hollow weir.

A condition of the coating process is preferably monitored, and one ormore of the previously mentioned operating parameters can be adjustedautomatically in response to the monitored condition.

In batch coating of tablets, the tablet bed can be relatively deep.However, in this invention, coating is a continuous process rather thanas a batch process. In a continuous coating process it is desirable tomaintain a relatively shallow tablet bed. In a preferred embodiment ofthe invention, excessive bed depth can be avoided by mounting the troughso that it tilts downward. That is, the trough is mounted so that itsexit end is lower than its inlet end. By adopting an appropriate degreeof downward tilt, depth of the tablet bed in the trough can bemaintained substantially constant along the length of the trough.

In a preferred embodiment of the invention, it is also desirable tofinish the inner surface of the trough so that it has a small amount ofroughness. A very smooth “mirror” finish, having an average surfaceroughness Ra of less than about 0.05 μm, is typical in a rotating drumcoater. However, in the case of a vibrating trough, depending on thefrictional coefficients of the tablets, a similar finish may not exhibitsufficient friction to achieve reliable tablet bed rotation. For thecoating apparatus of the invention, good results have been achieved fora variety of tablets, using a trough having an average surfaceroughness, Ra, in the range from approximately 0.2 μm to 0.8 μm.

One or more operating parameters can be adjusted automatically inresponse to a monitored process condition when air is caused to flowthrough the bed of pellets.

The invention has many advantages over conventional coating apparatusesand methods, especially in its capability of being scaled down, itscontrollability, and its ability to reduce damage to the product beingcoated. When the coating apparatus is scaled down, the reduced bed depthcan also reduce weight-induced damage to the product. Moreover, with theinvention, a batch process coating stage can be replaced by a continuouscoating stage at various throughput volumes, and the continuous coatingstage can be linked with other upstream and downstream continuousprocessing stages.

Other details and advantages of the invention will be apparent from thefollowing detailed description when read in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a continuous coating apparatusin accordance with a preferred embodiment of the invention;

FIG. 2 is a transverse cross-sectional view of the trough in thecontinuous coating apparatus having air plenums;

FIG. 3 is a schematic diagram illustrating the relationship of a spraynozzle to the bed of pellets in the trough of a continuous coatingapparatus corresponding to the apparatus of FIG. 1;

FIG. 4 is a schematic perspective view of a vibratory trough havingintermediate weirs;

FIG. 5 is a schematic perspective view, corresponding to FIG. 2,illustrating an alternative embodiment of the trough, having a hollow,perforated, intermediate weir;

FIG. 6 is a schematic diagram of monitoring and control components forthe continuous coating apparatus.

FIG. 7 is a schematic diagram of a horizontal vibratory trough, showingthe relationship between the trough and the tablet bed; and

FIG. 8 is a schematic diagram of a downwardly tilted vibratory trough,showing the relationship between the trough and the tablet bed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The term “pellets,” as used herein, includes solid, or at leastexternally solid, orally delivered pharmaceutical dosage forms,vitamins, candies, chewing gum, breath mints, animal feed, and the like.Usually, all of the pellets being coated by the method and apparatusdescribed herein will be of substantially the same size and composition.

The coating apparatus of FIG. 1 comprises three main sections: a feedersection 10, a coating section 12, and a discharge section 14.

The feeder section 10 can be any of a variety of mechanisms suitable fordelivering pellets to be coated to a desired location continuously at adefined average rate.

In FIG. 1, the feeder comprises a conveyor belt 16, on which pellets 18are carried to a chute 20. The rate at which the feeder delivers pelletsthrough the chute should be adjustable, and in the case of a beltconveyor, the feed rate can be adjusted by controlling the speed of adrive motor (not shown in FIG. 1) connected to a belt-driving drum,e.g., drum 22. The rate at which the feeder delivers pellets does notneed to be well-defined over a short interval such as a few seconds.However, the rate should be capable of being reasonably well-definedover a longer interval such as a minute, e.g., an average rate of 1000∀100 pellets per minute. Various alternative forms of feeding mechanismssuch as vacuum conveyors, screw conveyors, elevators, vibratingconveyors and other suitable conveyors can also be used.

Pellets drop through the chute 20 into an elongated, vibrating, trough24, at a location preferably adjacent one end of the trough, which willbe referred to as a trough “inlet.” In FIG. 1, the inlet is designatedby reference numeral 26. The trough may be open as in FIG. 1, or may beenclosed so that an air plenum is formed as will be described later. Inthe latter case, the inlet is a location adjacent a opening in theenclosure, which may be either controlled or uncontrolled. A weir 28, isdisposed at an outlet location or “outlet” 30, spaced longitudinallyfrom the inlet 26, and preferably located at the end of the troughopposite from the inlet 26. The weir has an upper edge 32, over whichpellets are discharged from the trough onto a collector 34, which can bein the form of a belt conveyor or hopper. The weir shown in FIG. 1 is inthe form of a segment-shaped plate. Alternatively, a ramp can be used,at the outlet of the trough, and the term “weir,” as used herein,includes ramps and other similar barriers over which pellets can bedischarged.

The upper edge of the weir 28 can be horizontal, or slanted, and neednot be straight. However, regardless of the shape of its upper edge, theweir establishes a maximum level for the bed 34 of pellets in thetrough. As will be described, the pellets in the bed are substantiallyfluidized, that is, made readily flowable, by the vibration of thetrough. Thus, as pellets are fed at a given rate to the inlet of thetrough, the bed slowly moves toward the outlet, and pellets aredischarged from the outlet at substantially the same rate. As vibrationtakes place, the upper surface of the fluidized bed of pellets willordinarily be disposed at an angle in the range of about 10 to 20degrees from the horizontal, depending on various factors such as themagnitude of the vibrations, air flow though the bed, the properties ofthe pellets, etc. However, in some cases, the surface of the bed canreach an angle of 45 degrees or more.

The trough 24 is flexibly supported on a frame 38 by a set of resilientsupports (not shown in FIG. 1) along one of its sides 40. These supportseffectively establish a pivot axis extending alongside the trough andsubstantially parallel to the direction of elongation of the trough. Thepivot axis, although preferably alongside the trough, can be inside thetrough, or above or below the trough.

The trough is connected by a set of arms 42, 44, 46 and 48, to a bar 50,which extends longitudinally along the trough on the side opposite tothe side on which the trough is pivoted. The bar 50 is mounted on theframe by springs 52, and connected to an energy-imparting source 54,which applies rapidly repeated mechanical impulses to the bar. In theapparatus shown, the impulses are applied vertically upward to the bar.However the impulses can be applied in any direction other than directlytoward the pivot axis of the trough, so that the impulses are applied asmoments about the pivot axis.

The repeated impulses must have sufficient intensity to fluidize the bedof pellets in the trough, and at the same time cause rotation of the bedof pellets so that pellets circulate continuously from the lowerportions of the bed to the surface, and then back to the lower portionsof the bed. The impulses must therefore be sufficient to overcome theeffect of gravity on the pellet bed, taking into account also the effectof any air flowing upward or downward through the bed. The movement ofthe bar 50 will ordinarily be in the range of 0.1 to 5 mm. The intensityof the impulses, of course, depends not only on their amplitude, i.e.,the range of movement of the bar 50, but also on the rate of movement ofthe bar, i.e., the slopes of the leading and trailing edges of eachimpulse when the amplitude of movement of the bar is plotted againsttime. The rotation and fluidization of the pellet bed are influenced notonly by the intensity of the impulses, but also by the frequency atwhich the impulses are applied, the frequency being preferably in therange from about 500 to 3500 Hz.

The energy-imparting source 54 can be any of a variety of mechanisms forproducing mechanical vibrations, such as electric motors havingeccentric weights mounted on their shafts, linear devices such aselectromagnetic vibrators, servomotors, etc. The energy imparting sourcecan be composed of plural energy-imparting units, if appropriatelysynchronized. Servomotors are preferable because they are easilycontrolled.

Preferably, one or more parameters of the impulse components, such asintensity, amplitude, direction, or frequency is adjustable, andsuitable adjusting means can be included as part of the energy-impartingsource. The adjusting means can be, for example, an electrical motorspeed control, a source of electrical pulses for operating anelectromagnetic vibrator, or any of a wide variety of equivalentadjusting devices. In the case of adjustment of direction, the adjustingmeans can be, for example, a mechanism for moving the energy-impartingsource itself or its output shaft or arm, or a mechanism for adjustingthe relative amplitudes of impulses delivered by two or more sourcescoupled together in order to adjust the direction of a resultantimpulse. The adjusting means can be adjusted manually, or automatically,with or without feedback from one or more sensors used to monitorconditions of the coating operation such as coating thickness, pelletfeed rate, and the like.

On the inside of the trough, a smooth mirror finish, that is, a finishhaving an average surface roughness Ra of less than about 0.05 μm,should be avoided because, for some tablets, it will not exhibitsufficient friction to achieve reliable tablet bed rotation. For thecoating apparatus of the invention, good results can be achieved for abroad variety of tablets, using a trough having an average surfaceroughness, Ra, in the range from approximately 0.2 μm to 0.8 μm.

Spray nozzles 56 and 58 are mounted on the frame and arranged to directfan-shaped spray patterns of liquefied coating material downward towardthe bed of pellets in the trough at intermediate locations between theinlet 26 and the outlet 30. The fan-shaped spray patterns are preferablyrelatively wide in the direction of the trough and relatively narrow inthe direction of the width of the trough.

The coating material can be any of a variety of known coating materials.In the case of pharmaceutical tablets, for example, the coating materialcan be a combination of a polymer such as polyvinylpyrollidone (PVP) orhydroxypropylcellulose (HPC), together with a pigment and an opacifiersuch as titanium dioxide (TiO₂), in a suitable vehicle such as water oran organic solvent, which partially evaporates as the spray approachesthe bed of pellets.

Various devices can be used to monitor the condition of the coatingapplied to the pellets. In FIG. 1, a sensor 60 is mounted on the frame38 above the pellet bed at a location downstream of the second spraynozzle 58 with respect to the direction of travel of the pellet bed inthe trough. The sensor is associated with a monitor (not shown in FIG.1), which can be a spectrometer for monitoring the thickness of thecoating on the pellets. A feedback signal from the spectrometer can beused to control the rate at which uncoated pellets are fed to the troughby the feeder section 10. Other conditions in the coating apparatus suchas temperature and humidity can be monitored, and the coating thicknessas well as these other conditions can be used, individually or incombination, to control operating parameters such as pellet feed rate,vibration amplitude, vibration intensity (which depends on bothamplitude and the rate of change of amplitude), vibration frequency,coating spray velocity and spray pressure. Where air is caused to flowthrough the bed of pellets, the temperature and flow rate of the air canalso be controlled in response to one or more monitored conditions.

FIG. 2 shows a version of the coating apparatus according to theinvention in which an array of air passages is formed in the trough, andupper and lower air plenums are formed by cooperation of the trough withenclosures respectively above and below the trough. These air plenumsare provided in order to maintain a flow of air through the air passagesand through the bed of pellets in the trough.

The trough 62 is a vibrating trough, supported on a set of arms similarto arms 42-48 in FIG. 1. One such arm, 64, is shown in FIG. 2, and issupported at one end on a mounting structure 66, which includes anelastomeric bar 68. The opposite end 70 of the arm 64 is connected to aenergy source (not shown) similar to energy source 54 in FIG. 1.

An array of air passages 72 is formed in the trough. The passages aresmaller than the pellets in the pellet bed 74 in the trough, andsituated so that most of the openings are below the top of the bed.

The trough is provided with a pair of flanges 76 and 78, which extendlongitudinally along its upper edges. Elastomeric sealing strips 80 and82 are clamped between the respective flanges and clamping strips 84 and86, and extend between flanges of upper and lower enclosures 88 and 90.

An upper enclosure 88 has an air passage 92, and the lower enclosure 90has an air passage 94. The pellet bed 74 is fluidized as a result ofvibrations imparted to the trough through the arms including arm 64,which vibrates up and down about an axis at the location of elastomericbar 68 in the directions indicated by the double-ended arrow 96. Themotion of the trough substantially fluidizes the bed of pellets, andcauses the pellets to circulate in a rotating path as indicated by arrow98. In the embodiment shown in FIG. 2, air passes into the upper plenumthrough passage 92, downward through the pellet bed 74 and passages 72into the lower plenum, and outward through passage 94 and blower 100.The upper and lower enclosures do not need to be sealed perfectly, andin the embodiment shown, for example, the arms on which the trough aremounted extend through slots in the lower enclosure 90. Pellets are fedinto, and discharged from, the vibrating trough, through suitableairlocks (not shown), for example, through rotary plate feeders of thekind illustrated in U.S. Pat. No. 6,416,261, dated Jul. 9, 2002, thedisclosure of which is incorporated by reference.

The flow of air through the bed of pellets provides for more uniformdistribution and drying of the coating material, and reduces the loss ofparticles of the spray from the spray nozzles, e.g., nozzle 102, to theatmosphere. Although in the embodiment shown, the flow of air throughthe pellet bed is maintained by drawing air outward from the lowerplenum by a blower 100, as an alternative, air can be forced into theupper plenum through passage 92. In some instances it may be desirableto maintain an upward flow of air, i.e., a flow of air into the pelletbed in the trough through the holes 72. In that case, blower 100 can bearranged to blow air into the lower plenum through passage 94, or ablower can be used to draw air outward from the upper plenum throughpassage 92. In any of these embodiments, the temperature and humidity ofthe air flowing through the bed of pellets can be controlled in order tomaintain proper coating conditions.

Although it is preferred to have both an upper air plenum and a lowerair plenum, advantages of air flow through the pellet bed can also berealized in a coating apparatus having only an upper air plenum, or onlya lower air plenum. Moreover, although it is desirable, but notabsolutely essential, to provide airlocks for feeding pellets to, anddischarging pellets from, the trough in the case in which an upper airplenum is used, airlocks are entirely unnecessary in the case of acoating apparatus having only a lower plenum.

As shown schematically in FIG. 3, a typical trough 104 has an arcuatecross-section. In this case, the arc is centered on a center line C(shown endwise as a point in FIG. 4). Pellets are discharged from thepellet bed in the trough over the upper edge of a weir.

As the trough is vibrated, the weir maintains the top of the fluidizedpellet bed substantially at a fixed position so that the center of thetop of the pellet bed remains at a constant height H, measured from thebottom of the trough. As mentioned previously, while vibration takesplace, the top of the bed will ordinarily be at an angle in the range ofabout 10 to 20 degrees from horizontal. However, depending on theintensity and direction and frequency of the vibrations of the trough,the pattern of circulation of the pellets in the trough, and otherfactors such as air flow and the nature of the pellets, the top of thebed can be disposed at an angle outside the 10 to 20 degree range.

As shown in FIG. 3, the bed, if at rest, would have a width W, measuredin a direction transverse to the direction of elongation of the trough.Because the pellet bed is coated in a vibrating trough, instead of in aconventional rotating drum, it is possible for the spray nozzle 106, orparts of the spray nozzle, to be located above the cylinder 108 thatwould be formed if the arcuate inner wall of the trough were continuedto form a complete cylinder. Thus, even in a coating apparatus having apellet bed with a relatively small cross-sectional area, it is possibleto position the spray nozzles far enough from the top of the bed thatthe sprayed coating material will dry sufficiently before it reaches thepellet bed, so that the solvents in the coating material do not damagethe surfaces of the pellets and impair coating quality.

In the case of an arcuate trough, the vertical distance D from thecenter of the top surface of the bed and the arc extension 108 is givenby the formula D=W²/4H. Thus, parts of the nozzle assembly can be at adistance greater than D from the center of the top of the pellet bed,that is, beyond the arcuate extension of the inner wall of the trough.

The trough 110 shown in FIG. 4 can be used in the coating apparatus ofFIG. 1, or in the coating apparatus of FIG. 2. This trough 110 has adischarge weir 112 at one end, a barrier 114 at the opposite end, whichis higher than the discharge weir, at the opposite end, and a series ofintermediate weirs 116. The height of each of the intermediate weirs ispreferably less than the height of the discharge weir 112, and is alsopreferably such that each of the intermediate weirs is entirelyunderneath the surface of the pellet bed when the trough is vibrating.The intermediate weirs block longitudinal movement of lower parts of thepellet bed and improve the uniformity of the residence time of eachpellet in the trough. It is possible to use a single intermediate weiror a plurality of intermediate weirs as shown.

One or more intermediate weirs can also be made hollow and provide withan array of holes for passage of air into or out of a trough. Forexample, as shown in FIG. 5, a trough 118, having a discharge weir 120at one end and a barrier 122 at the opposite end, is formed with anintermediate weir 124. The intermediate weir is hollow, forming a tunnel126 extending underneath the trough in a direction transverse to itslongitudinal direction. The wall 128 of the weir, which faces toward thedischarge end of the trough, is preferably, but not necessarily,vertical or nearly vertical, and has an array of air passages. Theopposite face of the intermediate weir (not shown in FIG. 5) can have asimilar array of air passages. The trough of FIG. 5 can be used in acoating apparatus having an upper air plenum, a lower air plenum, orboth as in FIG. 2. Because the air passages are in walls of the weirthat are either vertical or nearly vertical, the vertical component ofthe velocity of the air flow through the pellet bed is small and hasonly a negligible effect on the circulation of pellets resulting fromvibration of the trough. However, the air flow can be effective inmaintaining uniform coating conditions. Here, as in the embodiment ofFIG. 2, the air flow can be directed either inward through the airpassages to the pellet bed, or outward from the pellet bed through theair passages. The hollow weir also has the advantage of achieving a moreuniform residence time, and, of course, plural hollow weirs can beprovided in a trough.

In the control system depicted in FIG. 6, the spectrometer 60 (FIG. 1)is sensitive to the color of the coating on the pellets in the pelletbed passing underneath it. The color is a function of coating thickness.A monitor 130, e.g., a spectrometer, responsive to the sensor 60,generates a signal corresponding to coating thickness, and operates amotor control 132, which, in turn, controls the speed of a servomotor134 which operates the pellet feeder 10. If the coating becomes toothin, the rate at which pellets are fed to the trough can be reduced,and their residence time in the trough will be increased. Thus, auniform coating thickness can be maintained.

Other monitoring features and controls can be utilized. For example, thesignal from the monitor 130 can be used to control the vibration rate orintensity, or spray velocity, in addition to, or as an alternative to,controlling the pellet feed rate. The temperature and/or humidity of theexhaust air can also be monitored and used to control operatingparameters or combinations thereof, including air temperature, humidity,spray velocity, as well as pellet feed rate, and trough vibration rateor intensity. As shown in FIG. 6, the control 132 can also receiveinputs from an air temperature sensor 136 and a humidity sensor 138, anddelivers control signals to an air temperature and humidity control 140and a spray pump 142.

Although satisfactory results can be achieved with the trough of thecoating apparatus disposed horizontally, by tilting the trough downwardin the direction of travel of the pellets along the trough, so that theoutlet end of the trough is lower than the inlet end, the depth of thetablet bed can be made more nearly uniform along the length of thetrough thereby reducing the maximum depth of the bed. As shown in FIG.7, in the case of a horizontal trough, the depth of the tablet bed 144gradually decreases from the inlet end 146 of the trough toward theoutlet end 148. The shape of the tablet bed in the trough is affected bythe tablet feed rate, by the vibration of the trough, by drag due tofriction between the tables and the inner surface of the trough, and bygravity. Thus, the surface of the tablet bed slopes downward from theside at which the magnitude of trough vibration is maximum toward theopposite side of the trough, and also from the inlet end toward theoutlet end.

In the case of a downwardly tilted trough, as shown in FIG. 8, where theoutlet end 150 is lower than the inlet end 152, surface of the tabletbed 154 slopes downward from the side at which the magnitude of troughvibration is greater toward the opposite side. However, the slope of thetablet bed surface in the direction along the length of the trough isapproximately the same as the angle of downward tilt of the trough.Thus, the cross-section of the tablet bed is maintained nearly constantalong the length of the trough, the tablet bed is not excessively deepadjacent the inlet end of the trough, and more uniform coating of thetablets can be achieved. The uniformity of the depth of the tablet bed,of course depends on the tablet feed rate at the inlet end of thetrough. Therefore, the degree of tilt of the trough should be matched tothe tablet feed rate. Thus, the pellets should be fed at a rate thatmaintains the surface of the bed of pellets substantially parallel tothe direction of elongation of the trough.

Various other modifications can be made to the apparatus and methoddescribed above. For example, although the trough can be open as shownin FIG. 1, or provided with an upper air plenum as shown in FIG. 2, asan alternative, the trough can be the lower part of a closed channelthat is vibrated. The closed channel can even have a circularcross-section if sufficiently large that the spray nozzles are not tooclose to the pellet bed. However, in most cases, and especially wherethe apparatus is built on a scale suitable for continuous coating ofrelatively small quantities of pellets such as prescription drugs, ifthe trough is a lower part of a closed channel, the channel will have anon-circular shape, with a vertically elongated transversecross-section.

Whereas the continuous coating apparatus described above comprises alinear trough, various alternative configurations are possible. Forexample, the trough can have a toroidal configuration similar to that ofthe vibration mill described in U.S. Pat. No. 3,100,088, granted on Aug.6, 1963 to H. L. Podmore et al. and incorporated herein by reference. Inthat case, as described by Podmore et al. the impulses imparted to thetoroidal container by an eccentric weight on a centrally located motorwill cause circulating movement of pellets in the container as theytravel along the length of the trough. Another example is aconfiguration in which the coating apparatus is composed of a series ofvibrating troughs, located one above another, and arranged so thatpellets are fed from an upper trough to a next trough, and the pelletbeds travel in alternating directions in the respective troughs. Stillother trough configurations, such as an arcuate configuration, or ahelical configuration as in U.S. Pat. No. 5,067,431, granted on Nov. 26,1991 to Charles E. Heitmiller, can also be utilized.

The above modifications, and numerous other modifications, can be madeto the invention described without departing from its scope, as definedby the following claims.

1. A continuous coating apparatus for pellets comprising: an elongatedtrough having an inlet and an outlet separated from each other by alongitudinal distance along the direction of elongation of the trough; afeeder for continuously delivering pellets to be coated into the troughat a location adjacent the inlet and thereby establishing, in thetrough, a bed of pellets which travels longitudinally along the troughas pellets are delivered by the feeder; a weir disposed in the troughadjacent the outlet, the weir having an edge over which pellets aredischarged as pellets to be coated are delivered to the trough by thefeeder, and establishing the maximum level of a bed of pellets in thetrough; at least one spray nozzle disposed above the maximum level ofbed of pellets in the trough, each spray nozzle being arranged to directa spray of liquefied coating material toward pellets in the trough at anintermediate location between the inlet and outlet of the trough; amechanical energy-imparting source, connected to the trough, forvibrating the trough and thereby maintaining a bed of pellets therein ina substantially fluidized state, while rotating the bed so thatsubstantially all of the pellets become exposed to the spray ofliquefied coating material as they travel along the trough.
 2. Acontinuous coating apparatus according to claim 1, in which theelongation of the trough is linear, the trough is pivoted on an axissubstantially parallel to its direction of elongation, and themechanical energy-imparting source is connected to the trough at alocation spaced laterally from the axis, and arranged to apply impulsecomponents to the trough as moments about the axis, the impulsecomponents having sufficient intensity to rotate the bed of pellets inthe trough, whereby substantially all of the pellets reach the surfaceof the bed and are exposed to the spray of liquefied coating material attimes during their travel from the inlet to the outlet.
 3. A continuouscoating apparatus according to claim 1, in which the mechanicalenergy-imparting source is arranged to apply impulse components to thetrough, the impulse components having sufficient intensity to rotate thebed of pellets in the trough, whereby substantially all of the pelletsreach the surface of the bed and are exposed to the spray of liquefiedcoating material at times during their travel from the inlet to theoutlet, the apparatus including means for adjusting at least oneparameter of the impulse components from the group of parametersconsisting of intensity, amplitude, direction, and frequency.
 4. Acontinuous coating apparatus according to claim 1, including at leastone additional weir in the trough between the inlet and outlet.
 5. Acontinuous coating apparatus according to claim 1, including a series oflongitudinally spaced weirs in the trough between the inlet and outlet.6. A continuous coating apparatus according to claim 1, in which anarray of air holes is formed in the trough, and including an enclosurecooperating with the trough to provide an air plenum, and a blowerconnected to the air plenum for causing air to flow through the airholes and through the bed of pellets.
 7. A continuous coating apparatusaccording to claim 6, in which the blower is arranged to cause air toflow though the bed of pellets in the trough, and, from within the bedof pellets, outward from the trough through the air holes.
 8. Acontinuous coating apparatus according to claim 1, including at leastone additional weir in the trough between the inlet and outlet, theadditional weir being hollow and having walls facing the inlet of thetrough and the outlet of the trough, and at least one of the walls ofthe additional weir having an array of air holes, and an enclosurecooperating with the trough to provide an air plenum, and a blowerconnected to the air plenum for causing air to flow through the airholes and through the bed of pellets.
 9. A continuous coating apparatusaccording to claim 8, in which the blower is arranged to cause air toflow though the bed of pellets in the trough, and, from within the bedof pellets, outward from the trough through the air holes in theadditional weir.
 10. A continuous coating apparatus according to claim1, including a monitor for monitoring a condition of the operation ofthe coating apparatus, and a control, responsive to the monitor, foradjusting at least one parameter of the apparatus from the group ofparameters consisting of the rate at which the feeder delivers pelletsto be coated to the trough, air temperature, air flow, vibrationamplitude, vibration intensity, vibration frequency, vibrationdirection, liquefied coating material spray rate and spray pressure. 11.A continuous coating apparatus according to claim 1, in which themechanical energy-imparting source is arranged to apply impulsecomponents to the trough, the impulse components applying a force to thebed of pellets exceeding the force of gravity and being oriented andtimed to cause the bed of pellets in the trough to rotate continuously,whereby substantially all of the pellets reach the surface of the bedand are exposed to the spray of liquefied coating material at timesduring their travel from the inlet to the outlet.
 12. A continuouscoating apparatus according to claim 1, in which an array of air holesis formed in the trough, and including at least one enclosurecooperating with the trough to provide an air plenum, and a blowerconnected to the air plenum for causing air to flow through the airholes and through the bed of pellets, and including a monitor formonitoring a condition of the operation of the coating apparatus, and acontrol, responsive to the monitor, for adjusting at least one operatingparameter of the apparatus, from the group of parameters consisting ofthe rate at which the feeder delivers pellets to be coated to thetrough, air temperature, air flow, vibration amplitude, vibrationintensity, vibration frequency, vibration direction, liquefied coatingmaterial spray rate and spray pressure.
 13. A continuous coatingapparatus according to claim 1, in which an array of air holes is formedin the trough, and including at least one enclosure cooperating with thetrough to provide an air plenum, and a blower connected to the airplenum for causing air to flow through the air holes and through the bedof pellets, and in which the mechanical energy-imparting source isarranged to apply impulse components to the trough, the impulsecomponents applying a force to the bed of pellets exceeding theresultant of the forces applied to the bed by gravity and by air flowingthrough the bed, and being oriented and timed to cause the bed ofpellets in the trough to rotate continuously, whereby substantially allof the pellets reach the surface of the bed and are exposed to the sprayof liquefied coating material at times during their travel from theinlet to the outlet.
 14. A continuous coating apparatus according toclaim 1, in which the outlet end of the elongated trough is lower thanthe inlet end, whereby the trough is tilted downward in the direction oftravel of pellets in the trough.
 15. A continuous coating apparatusaccording to claim 1, in which the trough has an inner surface thatcontacts pellets in said bed of pellets, and in which said inner surfacehas an average surface roughness Ra in the range from approximately 0.2μm to approximately 0.8 μm.
 16. A process for continuously coatingpellets comprising: feeding pellets continuously into an elongatedtrough at a first location along the length of the trough, and therebymaintaining in the trough a bed of pellets continuously movinglongitudinally in the trough; directing a spray of liquefied pelletcoating material toward the bed of pellets in the trough at anintermediate location along the length of the trough; vibrating thetrough and thereby simultaneously substantially fluidizing the bed ofpellets disposed therein and causing the bed of pellets to rotate sothat substantially all of the pellets are exposed to the spray at timesduring their travel along the trough; and collecting coated pelletsdischarged over a weir in the trough at a discharge locationlongitudinally spaced from the first location, the intermediate locationat which the spray is directed toward the bed of pellets being betweenthe first location and the discharge location.
 17. The process of claim16, in which the elongation of the trough is linear, and the vibrationof the trough is carried out by applying impulse components to thetrough as moments about an axis parallel to the direction of elongationof the trough, the impulse components having sufficient intensity torotate the bed of pellets in the trough.
 18. The process of claim 16, inwhich the liquefied pellet coating material is sprayed toward the bed ofpellets in the trough by a spray nozzle located at a sufficient distancefrom the pellets that the coating material reaches the pellets in apartially dried condition such that solvent in the coating material doesnot damage the pellets.
 19. The process of claim 16, in which the bed ofpellets is moved along the trough over at least one additional weir inthe trough, the additional weir being spaced longitudinally from theweir at the discharge location.
 20. The process of claim 16, in whichthe bed of pellets is moved along the trough over at a series oflongitudinally spaced weirs in the trough in addition to the weir at thedischarge location.
 21. The process of claim 16, in which air is causedto flow through the bed of pellets while the spray of liquefied pelletcoating material is directed toward the bed of pellets.
 22. The processof claim 16, in which the bed of pellets is moved along the trough overat least one hollow weir in the trough, spaced longitudinally from theweir at the discharge location, and in which air is caused to flowthrough the bed of pellets and outwardly from the trough through anarray of openings in the hollow weir while the spray of liquefiedcoating material is directed toward the bed of pellets.
 23. The processof claim 16, in which a condition of the coating process is monitored,and, in response to the monitored condition, at least one operatingparameter is adjusted automatically in response to the monitoredcondition, the operating parameter being from the group consisting ofthe rate at which the pellets to be coated are fed to the trough, airtemperature, air flow, vibration amplitude, vibration intensity,vibration frequency, vibration direction, liquefied coating materialspray rate and spray pressure.
 24. The process of claim 16, in which airis caused to flow through the bed of pellets while the spray ofliquefied pellet coating material is directed toward the bed of pellets,in which at least one operating parameter is adjusted automatically inresponse to the monitored condition, the operating parameter being fromthe group consisting of the rate at which the pellets to be coated arefed to the trough, air temperature, air flow, vibration amplitude,vibration intensity, vibration frequency, vibration direction, liquefiedcoating material spray rate and spray pressure.
 25. The process of claim16, in which the outlet end of the elongated trough is lower than theinlet end, whereby the trough is tilted downward in the direction oftravel of pellets in the trough, and in which, in the feeding of pelletscontinuously into the trough at said first location, pellets are fed ata rate that maintains the surface of the bed of pellets substantiallyparallel to the direction of elongation of the trough.